1. Field of the Invention
This invention relates to digial to analog converters, and more particularly, to counter-type digital to analog converters for use with digital stereophonic audio signals, such as pulse code modulation audio signals from disc or tape players or audio adapters.
2. Description of the Prior Art
In a digital stereophonic audio reproducing apparatus employing pulse code modulated signals, the digital signals must be converted to an analog signal for reproduction. A digital to analog converter used with a digital stereophonic audio reproducing apparatus must convert a pair of PCM audio signals, one signal for each of the left and right stereophonic channels. Counter-type digital to analog converters have heretofore been utilized because of their superior performance over other types of digital to analog converters.
A counter-type digital to analog converter includes a current producing circuit portion in which a digital counter is loaded with the data word to be converted to an analog signal. The digital counter counts clock pulses until the count contained therein reaches a value corresponding to the data word. A constant current source portion supplies a predetermined constant current during the period which begins when the counter begins counting the clock pulses and ends when the value in the counter reaches the predetermined count. An integrating circuit portion is also included in a counter-type digital to analog converter in which an integrator is supplied with the constant current from the current producing circuit portion. The integrator produces an integrated voltage from the constant current. A low pass filter filters the integrated voltage generated by the integrator to derive the analog output signal.
Prior art digital to analog converters have two current producing circuit portions and two integrating circuit portions, one pair for each of the left and right stereophonic channels. The time period for converting the digital data words to analog output signals is generally selected to be relatively long so that a low frequency clock pulse can be utilized. Accordingly, the time for deriving the analog output signal from the integrating circuit is relatively short.
Prior art digital to analog converters are thus complicated and expensive because of the duplication of circuitry in the left and right stereophonic channel circuits. The level of the analog output signal at high frequencies is also reduced since the time period for deriving the analog output signal from the integrating circuit is relatively short, while the time period for converting the digital data words to analog output signals is relatively long.